This invention relates to a signal receiver for receiving multi-frequency signals sent from a push button dial telephone set or the like.
A prior art multi-frequency signal receiver is disclosed in U.S. Pat. Nos. 3,076,059 of the title Voice Frequency Signal System and No. 3,128,349 of the title Multi-frequency Receiver both assigned to Western Electric Co. Inc., U.S.A. The multi-frequency signal receivers disclosed in these patents generally comprise an input circuit, band elimination filters, amplitude limiters, band pass filters, voltage level detectors, OR logic circuits, AND logic circuit, an input signal continuation supervisory timer, and output gate circuits.
With a recent trend of using a push button dial telephone set, when using a multi-frequency signal receiver for a miniature telephone exchange set and for a push button telephone set, miniaturization and economization of the signal receiver are important problems. In the systems disclosed in said U.S. patents, the component element that hinders the miniaturization and economization comprises band elimination filters which are used to separate high and low frequency components. Such filter should be accurate because the highest frequency (941 Hz) of the low frequency components and the lowest frequency of the high frequency components (1209 Hz) are close to each other.